1. Field of the Invention
The present invention relates to magnetic resonance spectroscopic imaging technology, and more particularly to a side-band suppression method and a side-band suppression device.
2. Description of the Prior Art
As medical imaging develops, a new noninvasive examination method emerges in the magnetic resonance spectroscopic imaging field. In normal tissues, metabolites can be present in normal concentration. When pathological changes occur in the tissue, the concentration of the metabolites will change. Magnetic resonance spectroscopic imaging can obtain the metabolite concentration by observing the spectrum of the metabolites and further analyze the metabolite concentration to achieve the goal of disease diagnosis.
However, if there is imperfection in the design of the gradient system, the gradient switching can lead to mechanical vibration of the magnet, causing the main magnet field (B0 field) to oscillate over time, which leads to frequency modulation of the spectrum signal. In the spectrum, it appears as weak signal peaks, or side bands, in both sides of the normal signal peak. As the side-band intensity is proportional to the intensity of the modulated signal, the side bands will have a high intensity when the modulated signal has a high intensity and the high intensity side bands overlap and mix with the metabolite spectrum signal, causing difficulty in observation of the metabolite spectrum signal.
In order to suppress the side bands, the positive and negative gradient scanning method are commonly used in the prior art. Supposing the number of scans is N, wherein N is an even number, the gradient polarity used for the first N/2 scans is opposite to that used for the subsequent N/2 scans. For example, a positive gradient is used for the first N/2 scans and a negative gradient is used for the subsequent N/2 scans. Then the signals obtained from each scan are added up and the average value of the sum is calculated. The signal obtained by this scanning method is regarded as a signal without side bands. The theoretical foundation of this method is that the phase of the side-band signal is correlated to the polarity of the gradients and that changing the polarity of the gradients can change the phase of the side-band signal by 180 degrees, without affecting the phase of the metabolite signal. Two side-band signals with phase difference of 180 degrees can offset each other by addition. Theoretically the method can eliminate the side bands.
However, in real application, due to the existence of eddy current, the phase difference between the side-band signals in positive and negative gradient scanning is not exactly 180 degrees. Therefore the two side-band signals cannot completely offset each other. What is worse, the eddy current can also cause the initial phase deviation of the positive and negative gradient scanning signals, which leads to reduced signal-to-noise ratio (SNR) of the added up spectrum signal. For these reasons, the method provided by the prior art cannot effectively suppress the side bands.